Pressure sensitive adhesives (PSA), as defined by the Pressure-Sensitive Tape Council, are materials having aggressive and permanent tack, adhering with minimal contact pressure, requiring no activation by any energy source, having sufficient shear resistance under a load and sufficient cohesive strength to be cleanly removed from adherents. There are several classes of pressure sensitive adhesives, silicones being one of them.
Typically silicone PSAs cure either by peroxide radicals or through an addition reaction between a vinyl containing siloxane oligomer/polymer and a hydride containing siloxane oligomer. Peroxide curing tends to produce by-products that may affect the optical clarity of the adhesives. Accordingly, peroxide curable PSAs are not suitable for use in construction of electronic displays, such as cell phones, GPS navigation, and television displays, where optical clarity is essential. Compared with peroxide curable PSAs, addition curable silicone PSAs do not normally contain by-products that affect optical clarity of the adhesives, thus being suitable for use in construction of multi-layer laminate electronic displays.
Addition curable silicone PSAs are known in the art and have been described for example in U.S. Pat. Nos. 3,983,298; 4,774,297; 4,988,779; 5,169,727; 5,248,739; 6,466,532; 5,576,110; and JP 8209104. However, these prior art addition curable PSAs relied on multi-component blends to achieve required adhesive properties of tack, peel adhesion, and cohesive strength. As such, the requisite crosslinking has been unpredictable and the adhesive properties of the PSAs upon cure have been unstable.
Attempts have been made to solve the problems associated with the prior art addition curable silicone PSAs. Illustratively, US 20050113513 to Griswold discloses an addition curable silicone PSA composition that contains a self-crosslinkable component and a hydrosilation catalyst. According to Griswold, the self-crosslinkable component can be made by (1) condensing an aromatic soluble silicone resin or resinous copolymer with an alkenyl containing polydiorganosiloxane, and (2) reacting the condensation reaction product with a silylating agent to cap the silanol groups remaining from the condensation reaction to form silicon-bonded hydrogen groups using as an example 1, 1,3,3-tetramethyldisilazane.
However, one drawback of the process is that after the condensation reaction, not all remaining silanol groups are accessible to silylation because of steric hindrance. Further, as shown in examples 4 and 5 of the '513 publication, after the addition reaction, the silicon-hydride concentrations were high due to the hydride group being sterically hindered for the hydrosilation reaction with the vinyl groups on the polymer/oligomer. This limitation prevents an effective means for controlling crosslink density and therefore cohesive strength for a pressure sensitive adhesive. Furthermore, the residual silicon-hydride groups affect the stability of pressure adhesive properties such as peel adhesion and tack upon storage. In addition, residual hydride after hydrosilation cure can result in out gassing in a laminate construction being observed as bubbles.
Several other attempts to resolve the defects in the prior art were also unsuccessful. Accordingly, there is a continuing need in the industry for addition curable silicone pressure sensitive adhesive compositions that have controllable crosslinking and stable post-cure adhesive properties.